The chick cartilage somatomedin bioassay is clinically significant because of its usefulness in problems involving growth and development. However chick cartilage sulfation may be affected by inhibitory factors, proteoglycan precursors and hormones. These serum factors may introduce errors into the chick assay system. We will continue to study the relationship of T4, T3 and somatomedin in the serum of developing chick embryos. Currently, we do not have adequate data for a significant statistical evaluation of these results. Previous work indicates that physiological concentrations of T4 and T3 do not alter the somatomedin activity of normal rat serum while higher concentrations enhance the potency of serum. These studies will be extended to evaluate the direct effect of T4 and T3 in human serum in the presence and absence of somatomedin. The thyroid hormones, through their effects on maturation, may alter the cartilage responsiveness to sulfation factors. Therefore to determine the importance of the thyroid on chick embryo cartilage in vivo eggs will be injected with sub-lethal doses of T4, T3 or thiouracil and cartilage uptake determined during the following stages of development. In addition, we will determine whether the ratio Ch-4-SO4/Ch-6-SO4 in chick cartilage is altered by T4 and T3 treatments. BIBLIOGRAPHIC REFERENCE: In vivo and in vitro influence of thyroid hormones on somatomedin activity. T. Gaspard, R. Wondergem, M. Hamamdzic, and H.M. Klitgaard. Endocrine Society, 1977.